Hair styling appliance

ABSTRACT

A hair styling apparatus includes a plurality of heater electrodes which heat one or more hair styling heaters, a power source for powering the plurality of heater electrodes and a controller configured to control powering of the plurality of heater electrodes from the power source. The plurality of heaters includes a first subset and a second subset of heaters. The controller is configured to, in a first mode of operation; control the power source so that the first and second subsets are not simultaneously powered.

FIELD OF THE INVENTION

This invention relates to hair styling appliances, in particular lowvoltage, for example battery operated devices.

BACKGROUND TO THE INVENTION

There are a variety of apparatus available for styling hair. One form ofapparatus is known as a straightener which employs plates that areheatable. To style, hair is clamped between the plates and heated abovea transition temperature where it becomes mouldable. Depending on thetype, thickness, condition and quantity of hair, the transitiontemperature may be in the range of 160-200° C.

A hair styling appliance can be employed to straighten, curl and/orcrimp hair.

The temperature range required, user expectations with regard to thetime to heat-up, thermal control, and other factors combine to driveexisting hair styling appliances to employ mains power for theheater(s).

In WO2014/001769 and GB2503521 to the present applicant, a hair stylingappliance including a battery power source for at least one heater istaught.

The inventors have realised that further improvement in the use of abattery power source is possible.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a hairstyling apparatus comprising: a plurality of heater electrodes whichheat one or more hair styling heaters, the plurality of heaterscomprising a first subset and a second subset; a power source forpowering the plurality of heater electrodes and a controller configuredto control powering of the plurality of heater electrodes from the powersource, wherein, in a first mode of operation, the controller isconfigured to control the power source so that the first and secondsubsets of the plurality of heaters are not simultaneously powered. Thepower source may be a battery source. Alternatively, the power sourcemay be mains power.

Typically, in this first mode of operation, the controller controlspower delivery so that power is delivered to the first and secondsubsets in a time interleaved manner, preferably multiple times persecond. Interleaving the power delivery in this way offers a number ofadvantages. For example, if the first subset of heater electrodes isassociated with a first hair styling heater and the second subset ofheater electrodes is associated with a second hair styling heater, thenthe controller can control the heating of the first and second hairstyling heaters so that they are both heated, at the same time, torespective desired operating temperatures (which may be the same).Similarly, if the first and second subsets of heater electrodes areassociated with different parts of one hair styling heater then thecontroller can control the heating of the different parts of the heaterso that they are both heated, at the same time, to a desired operatingtemperature. This is possible as such hair styling heaters (and otherkinds of heaters) normally have a relatively high thermal inertia sothat they do not cool down quickly once power is removed from the heaterelectrodes.

Interleaving of the driving of the electrodes in this manner alsoreduces the current drawn from the power source. When the power sourceis a battery, reducing the current draw is important as this reduces theenergy lost in the internal resistance of the battery: P=I²R, where I isthe current drawn and R is the internal resistance. Hence operating inthis first mode of operation provides the most efficient heating of theheaters. Of course, in other modes of operation heaters from the firstand second subsets may be powered simultaneously, for example, if theload is very high.

The controller may be configured to select the first and second subsetsso that a total current drawn by each of the first and second subsets isbelow a predetermined current threshold. This is particularly useful fora battery power source because keeping the current draw below thethreshold may prevent the battery from overheating (due to the abovedescribed I²R losses). The predetermined current threshold may beequivalent to a multiple (e.g. 1.5) of the current draw for a singleheater electrode.

In other words, the controller is configured to control powering of theheater electrodes from the power source. The first mode of operation ofthe controller (there may only be the one mode) comprises limiting thetotal number of heater electrodes that may be simultaneously poweredsuch that a predetermined current limit is not exceeded. The fact acurrent limit is imposed means that the controller is configured toprevent all the heater electrodes being powered at the same time. Thiscurrent limit may be deemed a nominal current draw.

The plurality of heater electrodes may be divided into discrete subsets.Thus no heater electrodes are in both the first and second subsets.Alternatively, the first and second subsets may have some (but not all)heater electrodes in common. It will be appreciated that there may bemore than two subsets of electrodes, e.g. three or even four, dependingon the overall number of heater electrodes within the apparatus. Eachsubset may comprise one or more electrodes.

The controller may be configured to switch the power source between thefirst and second subsets, for example multiple times per second. Thismay be done, for example, to maintain the heater at a desired operatingtemperature. The controller may comprise a heating cycle in which itcycles through all of the subsets of powerable heater electrodes,determining if power may need to be applied. If not, the controller mayopt to retain power to the currently powered subset, switch to another,or opt to power none of the heater electrodes if heater plates (or zoneson a heater plate) are at a preferred operating temperature. Theswitching frequency between each subset may be in the order of tens,hundreds or thousands of cycles per second. Typically the heating cyclewill have a period of between 100 μs and 500 ms. The first and secondsubsets may be powered in anti-phase, i.e. one subset is off when theother subset is on. In such anti-phase operation there may however beperiods in which none of the electrodes are powered.

The controller may be configured to alternate between the first mode ofoperation and a second mode of operation in which the first and secondsubsets of the plurality of heater electrodes are simultaneouslypowered. For example, simultaneous heating may take place during theinitial heat up from power on. In this second mode of operation, thenominal current draw is exceeded temporarily. When operating in thissecond mode, the controller may switch the power to both subsets ofheater electrodes such that there are: 1) overlapping periods in whichpower is simultaneously supplied to heater electrodes in the first andsecond subsets; 2) periods in which power is supplied to heaterelectrodes of just one of the first and second subsets; and 3) periodsin which no power is supplied to heater electrodes of the first andsecond subsets. The controller may control the duration of theoverlapping periods so that they reduce with time from, for example, aninitial switching on time. The controller may reduce the duration of theoverlap either in response to a sensed condition or based on pre-storeddata defining the switching sequence.

The controller may be configured to switch from the second mode ofoperation to the first mode of operation in response to a controlsignal. The control signal may be that a predetermined amount of time inthe second mode of operation has elapsed. The predetermined amount oftime may be based on predetermined characteristics of the battery powersource and may be a few seconds. The controller may then continue tooperate in the first mode of operation for a further period of time (forexample 30 seconds or more) after which periods of operating in thesecond mode may be possible.

Where the power source is a battery source, the hair styling apparatusmay further comprise a battery temperature sensor which senses thetemperature of the battery source and which sends a battery temperaturesense signal to the controller. In some embodiments a batterytemperature sensor may be integrated into the hair styling apparatus,however in other embodiments the battery power source may comprise anintegrated temperature sensor having a connection coupleable to thebattery temperature sense input.

The battery temperature sense signal may be compared to a batterytemperature threshold and the control signal may be generated by thecontroller or at the sensor when the battery temperature sense signal isgreater than the battery temperature threshold. Alternatively, thecontrol signal may be generated when the battery temperature isincreasing at a rate such that a threshold value is predicted to beexceeded. The battery temperature threshold may be in the range of 60 to80 degrees C., more preferably 70 degrees C.

There may also be temperature sensor sensing ambient temperature. Saidcontrol signal may be generated by the controller or the sensor when thesensed ambient temperature is below a threshold ambient temperature. Theambient temperature threshold may be in the range of 25-35 degrees C.,more preferably 25 or 33 degrees C.

In other words, the controller may be configured to limit a duration inwhich the subsets of heater electrodes are simultaneously powerable bythe power source. This may also be dependent on the battery temperatureor ambient temperature.

The hair styling apparatus may comprise a first arm having a firstcontacting surface and a second arm having a second contacting surface,wherein the arms are moveable between a closed position in which thefirst and second contacting surfaces are adjacent and an open positionin which the first and second contacting surfaces are spaced apart. Thefirst arm may comprise a first hair styling heater having a plurality ofheater electrodes. The first arm comprises a first hair styling heaterand the second arm comprises a second hair styling heater and each hairstyling heater comprises at least one heater electrode. In thisarrangement, the plurality of electrodes comprises at least one on eacharm. A plurality of electrodes includes two electrodes. Where there areonly two electrodes, e.g. one on each arm or a single heater with twoelectrodes, the first subset may comprise the first electrode and thesecond subset may comprise the second electrode.

The hair styling apparatus may further comprise a touch sensitive switchconfigured to enable or disable the hair styling apparatus. It will beappreciated that the touch sensitive switch can be used on its own as aseparate invention as well as in conjunction with the different poweringmodes of operation.

For an apparatus having a pair of arms as described above, the touchsensitive switch may be located on the first or second contactingsurface. When the arms are in the closed position, the touch sensitiveswitch may be deactivated to prevent unintended activation of theswitch. In use, a user may activate the touch sensitive switch bypressing on or otherwise contacting said touch sensitive switch for atleast a predetermined duration of time. This activation may bedetermined by the switch or the controller and the controller may enableor disable the hair styling apparatus responsive to said determining.

The or each heater may comprise a heater plate which is mounted on athermally insulating support structure. It will be appreciated that thethermally insulating support structure can be used on its own as aseparate invention as well as in conjunction with the different poweringmodes of operation and/or touch sensitive switch.

The heater plate may comprise at least one recess which cooperates witha corresponding projection on the thermally insulating supportstructure. The recess and projection may be L-shaped. Other mechanismsfor mounting the heater plate on the support may be used. The thermallyinsulating support structure may be resiliently mounted within an arm ofthe hair styling apparatus. For example, a spring mechanism may be used.Such a resilient mounting allows the heater plate to move relative tothe casing of the arm during styling, allowing the plates to retaincontact with varying thicknesses and changes in the profile of hairclamped between opposing pairs of styling surfaces on the heater plates.

According to another aspect of the invention there is provided a methodof controlling a hair styling apparatus comprising a plurality of heaterelectrodes which heat one or more hair styling heaters, the plurality ofheaters comprising a first subset and a second subset; the methodcomprising: controlling powering of the heater electrodes so that thefirst and second subsets of the plurality of heaters are notsimultaneously powered.

According to another aspect of the invention there is provided acontroller for a hair styling appliance, wherein the controllerconfigured is configured to implement the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows a first example of a hair styling appliance in whichembodiments of the invention may be employed;

FIG. 2 shows a schematic block diagram of a hair styling appliance ofthe type illustrated in FIG. 1;

FIG. 3 shows a plan view of an embodiment of a hair styling heater foruse in the hair styling appliance of FIG. 1;

FIGS. 4a and 4b show timing diagrams illustrating example duty cycles ofheating electrodes driven, for example, by the control system in FIG. 2;

FIG. 5 shows a variant of the schematic block diagram of FIG. 2;

FIGS. 6a to 6d show timing diagrams illustrating example duty cycles ofheating electrodes driven, for example, by the control system in FIG. 5;and

FIG. 7 shows a cross sectional view of the hair styling appliance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts an example of a typical hair straightener 1. The hairstraightener 1 includes first and second arms each comprising an armmember 4 a, 4 b and heatable plates 6 a, 6 b coupled to heaters (notshown) in thermal contact with the heatable plates. The heatable platesare substantially flat and are arranged on the inside surfaces of thearms in an opposing formation. During the straightening process, hair isclamped between the hot heatable plates and then pulled under tensionthrough the plates so as to mould it into a straightened form. The hairstraightener may also be used to curl hair by rotating the hairstraightener 180° towards the head prior to pulling the hair through thehot heatable plates.

Also shown in FIG. 1 is touch sensitive switch 5070 which is used topower the hair styling appliance on and off. The switch may beimplemented as a capacitive touch switch comprising an electrode placedbehind the plastic casing of the hair styling appliance. This obviatesthe need for a mechanical switch. In variants a resistive touchsensitive switch may also be used, or a piezo touch switch. As shown inFIG. 1, the touch sensitive switch 5070 is positioned on the inside ofan arm facing the other arm. This means that the switch can only bepressed when the arms are spaced apart to prevent a user accidentallytouching the switch and unintentionally turning the hair stylingappliance on. Further, should the hair styling appliance be placed inluggage, such as a handbag, if a user rummages around in the bag for anitem, it also prevents any accidental pressing of the switch. As afurther safety mechanism, in embodiments the switch may also bedeactivated (or the power supply configured to prevent activation of theappliance) when the arms of the hair styling appliance are closedtogether.

It will be appreciated that a hair straightener is just one example of ahair styling appliance and a skilled person would implement the variousembodiments of the invention without difficult into other hair stylingappliances such as a “crimping iron” for crimping hair or a hair stylingappliance for curling hair.

FIG. 2 shows a block diagram of a power/control system 500 for a hairstyling appliance incorporating a heater 300. The system comprises a lowvoltage power supply 504 deriving power from a 12v lithium ion battery505 and/or a mains power supply input 502, which is used to charge thebattery 505 via an AC to DC converter 503 which may be external orinternal to the appliance. Power supply 504 may be configured to provideapproximately 100 watts per heater; the heater resistance when hot maybe selected accordingly—for example at 12v a current in the range 5-10amps may be delivered to a heater with a resistance in the range 1-2ohms. The resistance may be scaled accordingly as the design voltageincreases or decreases (changing as the inverse square of the voltage).

Power from power supply 504 is provided to a power control module 514,which in turn powers the one or more heaters 516. Power control module514 may employ one or more power semiconductor switching devices toprovide pulse width modulation control of the (DC) voltage from powersupply 504 to heaters 516. Thus a high percentage on-time duty cycle maybe employed during the initial, heating phase and afterwards the on-timeduty cycle may be reduced and controlled to control the temperature(s)of the heaters 516.

Power from power supply 504 is also provided to amicrocontroller/control means 506 coupled to non-volatile memory 508storing processor control code for a temperature control algorithm, andto RAM 510. The skilled person will appreciate that any of a wide rangeof different control algorithms may be employed including, but notlimited to, on-off control and proportional control. Optionally thecontrol loop may include a feed-forward element responsive to a furtherinput parameter relating to the hair styling appliance, for example touse the operation of the apparatus to improve the temperature control.An optional user interface 512 is also coupled to microcontroller 506,for example to provide one or more user controls and/or outputindications such as a light or audible alert. The output(s) may beemployed to indicate, for example, when the temperature of the heatingplate has reached an operating temperature, for example in a region 140°C.-185° C.

Microcontroller 506 is also coupled to one or more optional temperaturesensors such as thermistors 340. However, as previously mentioned, thetemperature of a heating element may be sensed from its resistance andthus embodiments of the system include a current sense input 515 tomicrocontroller 506 sensing the current provided to a heater, forexample via a current-sense resistor connected in series with theelectrode. A predetermined calibration of resistance against temperaturefor an electrode may be stored in non-volatile memory 504 and in thisway the printed track may be employed as a temperature sensor.

In the illustrative embodiment of FIG. 2, the touch sensitive switch5070 is shown coupled to the low voltage PSU 504 to remove the need forthe microcontroller to be permanently powered up. When off, the lowvoltage PSU 504 monitors for a change in capacitance of the touchsensitive switch indicating that a user has pressed the switch. The lowvoltage PSU then powers up the hair styling appliance. When on, the lowvoltage PSU 504 again monitors for a change in capacitance of theswitch, then powering down the hair styling appliance. To power up anddown, it may be necessary for a user to press on the touch sensitiveswitch for a minimum period of time before the PSU fully registers thepress as a valid request to power on/off. This eliminates any accidentalpower up, or when styling, any accidental touching of the touchsensitive switch. A user may, for example, be required to press thetouch sensitive switch for two, three, four or five seconds, or longer.In variants two touch sensitive switches may be used: one for turningthe styling appliance on, another for turning the styling appliance off.

Providing such a touch sensitive switch on a hair styling applianceprovides several advantages. Firstly, for battery powered products, ifthe hair styling appliance is carried around in luggage, it prevents theappliance being accidentally knocked on by other items in the luggage.Further, it also improves the aesthetic appearance of the product,eliminating the need for additional components on the surface of thehair styling appliance.

In this embodiment the touch sensitive switch is shown coupled to thelow voltage PSU. In variants such a switch may be coupled to themicrocontroller, although in such variants it will then be appreciatedthat the microcontroller may then need to be permanently powered topermit detection of a press of the touch sensitive switch. In othervariants a dedicated circuit may also be used.

This touch sensitive power switch may be applied to any of theembodiments described herein and also as an adaption to otherwisestandard devices.

Each heater plate may be powered by a heater electrode. Depending uponthe thickness of the heater plate, lateral conductivity within the platemay not be sufficient to give the desired results with a single heaterelectrode. Accordingly, an example of a heater plate is illustrated inFIG. 3 which may form the heatable plates 6 a, 6 b of the hairstraightener of FIG. 1. The heater plate 300 may be provided with aplurality of separately controllable heating zones 300 a, b, each with arespective electrode 330 a, b and thermistor 340 a, b. Connections tothese are brought out, for convenience, to one edge of the heater plate;a broadened track region 332 is provided for the electrode further fromthe connection point to reduce heating in the connection path. Each ofthe electrodes is provided with a separate control loop controlled bythe temperature sensed by the respective thermistor. In embodiments morethan 3 zones may be provided.

The heater used in the various embodiments described herein may beformed as described in WO2014/001769 and GB2503521 which areincorporated by reference. Thus, the heater may comprise an aluminiumheater plate of thickness of order 1 mm, bearing a plasma electrolyticoxide (PEO) coating of aluminium oxide of thickness less than 100 μm,for example in the range 5-15 μm.

The hair styling appliance comprises a plurality of electrodes. As shownin FIG. 1, there may two heater plates, each with their own electrodeand thus there are two electrodes. Alternatively, only one arm of theappliance shown in FIG. 1 may comprise a heater plate but this heaterplate comprises at least two, possibly more electrodes. There may alsobe multiple heater plates each having multiple electrodes.

The power to the plurality of heater electrodes may be independentlycontrolled. For example, for an appliance having two arms, each with aheater and one heater electrode for each heater, FIGS. 4a and 4b showthe Voltage against Time for each heater electrode. In this exampleillustration, the microcontroller 506 uses pulse width modulation (PWM)power control to control the supply of power to the two heaterelectrodes using the power control block 514. In pulse width modulationpower control, the “on time” of the power signal within a sequence ofPWM periods (here labelled ‘d’ and also referred to in other parts ofthis document as heater cycles) is varied in order to vary the amount ofpower delivered to each heater electrode. Typically, the PWM period /heater cycle may be between 100 μs and 500 ms. It will be observed fromFIGS. 4a and 4b that initially both heater electrodes are poweredsimultaneously until the desired operating temperature is reached. Ahigh percentage on-time duty cycle within any given period ‘d’ may beemployed during the initial heating phase; afterwards the on-time dutycycle may be reduced for each heater so as to retain the hair stylingheater at a desired operating temperature. Each heater may be controlledindependently to stabilise the temperature of each heater at the desiredoperating temperature. Accordingly, the draw is not exactly the same forboth heater electrodes. Nevertheless, it can be observed from FIGS. 4aand 4b that there may be sustained periods of a maximum current draw asa result of both heaters being powered simultaneously.

When powered by battery, a high current draw, such as from driving bothheaters simultaneously, may lead to the battery power source heating upto an unacceptably high temperature. This may be exacerbated if bothheaters are simultaneously driven for extended periods of time. Thecurrent draw when simultaneously driving the heaters, combined with adesire to conceal the battery power source means that heat dissipationmay become an important factor in the construction of such a hairstyling appliance.

FIG. 5 shows a variant of the schematic block diagram of FIG. 2 withmodified microcontroller/control means and power control module 514 b.The reference numbers in common are used in both systems and thus anydescription applies equally to both.

The microcontroller switching control signal 708, labelled ‘temp/powercontrol’ in FIG. 5 may comprise multiple outputs, one for each heaterpower switch to be controlled. The embodiment shown in FIG. 5 comprisestwo heater electrodes, one on each of the two heater plates. Twooutputs, one to activate the first heater electrode via the first powerswitch 702 and the second to activate the second heater electrode viathe second power switch 704 are present on the power control module. InFIG. 5, further heater electrodes may also be driven. These may bepresent, for example, in a multi-zoned heater variant. Dotted arrowlines to the heaters 516 show optional connections to such additionalheater elements.

In variants the switching control signal may be a serial data connectionor encoded such that the control system can scale to independentlycontrol multiple heater elements. This may be particularly useful inembodiments having multiple heating zones (two or three per heater forexample) and where the number of outputs from the microcontroller may belimited. Alternatively the microcontroller may have multiple outputs,one for each power switch. The optional decode block 706 in the powercontrol module 514 b decodes the signal received from themicrocontroller and splits this out into separate drive signals toactivate the power switches 702 and 704. In variants incorporatingmultiple heating zones on each heater plate the signal may be decodedinto more outputs, one for each zone.

Battery power source 505 in FIG. 5 may further incorporate a batterytemperature sensor 5050, such as a thermistor. The battery temperaturesensor 5050 provides a battery temperature sense signal 5051 coupled tothe microcontroller 506. The battery temperature sense signal may befactored into the temperature control algorithm and powering of theheater plates. It will be appreciated however that such a feature isoptional.

In the embodiment shown in FIG. 5, the battery temperature sensor manyalso be used as part of the safety shutdown 520. As set out above, thestyling appliance may incorporate one or more safety shut down circuits520 coupled to the one or more heater electrodes and/or temperaturesensors 340 to monitor the heater temperature and electronically shutdown the power supply to the heater should overheating be detected. Inembodiments, this may be extended to also prevent overheating of thebattery. In embodiments safety shut down circuit 520 controls a guardtransistor 522, as illustrated a power MOSFET, which removes power fromthe power control block on detection of a potential fault. Guardtransistor 522 may be provided either before or after power controlblock 514 a. In normal operation this device is always on; the devicemay be selected such that when power is removed from the transistor itswitches off, thus failing safe, for example by employing anenhancement-mode device. Such control and safety shut down is applicableto all the embodiments described herein.

The battery temperature sensor 5050 (or another battery temperaturesensor) may additionally or alternatively be used to control poweron/off of the hair styling appliance. The generated signal B_(Tsense)5051 is fed into the microcontroller 506 and may be used to providebattery temperature information for use in a safety mechanism to shutdown the styling appliance, or stop power delivery to one or more heaterelements/plates if the battery temperature exceeds a battery thresholdtemperature. Thus, the microcontroller/control means may be arrangedsuch that power is only supplied to the heating elements/plates when thetemperature sensed by the battery temperature sensor is below a batterythreshold temperature. In embodiments, this threshold temperature may bea value in the range of 60-100° C., for example 70° C. However it willbe appreciated that the operational threshold temperature may bedependent on the particular construction (packaging, chemicalformulation for example) of the particular battery used. Techniques suchas active cooling of the battery pack, or heat transfer means such as aheat sink, may often be insufficient to retain the battery pack withinits preferred safe operating range.

Following deactivation of the heater plates, the control system mayprevent the hair styling appliance from being used again until thebattery temperature has fallen below either the battery thresholdtemperature at which power down was previously initiated, or below alower ‘reactivation’ temperature which would be set to a temperaturebelow the battery threshold temperature.

Another technique that may be used to prevent heat build-up in thebattery power source is to slow the rate of heating by throttling themaximum current delivered or using higher resistance heating elements.However, adopting such a technique may mean that the temperature of aheater plate cannot be changed very rapidly, which may lead to a poortransient response.

Further, an ambient temperature sensor, such as temperature sensor 5060in FIG. 5 may be used to monitor the ambient temperature (i.e. thetemperature surrounding the hair styling appliance) and prevent powerdelivery to the heater plates if an ambient temperature threshold isexceeded. An ambient temperature sense signal A_(Tsense) 5061 is thengenerated and fed into the microcontroller 5061. In embodiments, thisambient threshold temperature may be a value in the range of 25-35degrees C., for example 25 degrees C. or 33 degrees C. Such ambienttemperature sensing may be used as a further safety mechanism to protectagainst overheating of the hair styling appliance.

This is particularly useful in warmer environments in which the batterywhich may heat up too fast (such as outside in hot climates, or in a hotindoor environment). A user may then be preventing from turning on thehair styling appliance until the ambient temperature has reduced. Thus,the microcontroller/control means may be arranged such that power isonly supplied to the heating elements/plates when the sensed ambienttemperature is below an ambient threshold temperature.

In one or both instances above, if either the ambient or batterythreshold temperatures are exceeded; visual or audio feedback may beprovided to the user to indicate that the device has entered a safetymode or indicate the temperature status.

In a first control mode the microcontroller/control means in FIG. 5 isconfigured to lower the maximum current draw by operating the heaters inanti-phase. This means that in an embodiment having two heaterelectrodes (one for each heater), only one heater element may be poweredat a time in the embodiment in FIG. 5. Operating in anti-phase, theremay also be periods where both heaters electrodes are off, such as whenboth heater plates are at a desired operating temperature. Given acurrent draw of ‘I’ for one heater arranged to heat an entire heaterplate, when two heaters are powered simultaneously to heat two heaterplates, the current draw may be approximately ‘2I’. Using thisconvention, the maximum current draw is limited to the current draw fordriving one heater (i.e. ‘I’, the current draw for one heater arrangedto heat an entire heater plate). This means that the controller may beconfigured to prevent all (both in the embodiment of FIG. 5) heaterelectrodes being powered at the same time.

FIGS. 6a and 6b show a graph for each heater in an appliance having twoheaters and operating according to the preceding paragraph. As before,the microcontroller 506 uses pulse width modulation power control tocontrol power delivery to the two heaters. It will be observed fromFIGS. 6a and 6b that both heater electrodes are now not powered at thesame time within each heating cycle (here labelled ‘c’). The dottedlines between the Figures show instances of one heater starting orstopping—note there is no overlap. In a heating cycle (‘c’), each of theheaters below temperature are powered in a sequence which may be fixedor determined by the microcontroller, but only one at a time. As before,the heating cycle may be between 100 μs and 500 ms and thus thecontroller rapidly switches the delivery of the power between the twoheaters such that, as far as the user is concerned, both heaters appearto be heating up simultaneously.

As will be apparent to those of ordinary skill in the art from FIG. 5,the controller 506 controls this delivery of power to the first andsecond heaters by generating control signals 708 that cause thedecode/drive enable unit 706 to open and close the switches 702 and 704.In some cases, the control signals 708 may be directly used to controlthe switching of the switches 702 and 704.

The microcontroller may implement a control algorithm configured toallocate equal percentages of a heating cycle ‘c’ to each heater, forexample 50% of the time. Typically this may be the case when a userpowers on the appliance to heat both heater plates to the desiredoperating temperature evenly and as fast as possible. However, in theevent that one heater plate heats up slower than the other, a higherportion of time in any given period/heating cycle ‘c’ may be allocatedto the cooler heater plate. Furthermore, in the event one heater platecools faster than the other when placed about a quantity of hair, themicrocontroller, in response to a temperature dependent sense signal,may act accordingly to allocate a higher portion of heating time in anyone heating cycle to power the heater in the cooler heating plate.

In some embodiments of the hair styling appliance, there may be multipleheating zones on each heater plate, as shown in FIG. 3 for example andalso shown in GB2477834, herein incorporated by reference. Each heatingzone may comprise a separate heater electrode arranged to heat a portionof the heater plate. In such embodiments, it may then be permissible tosimultaneously heat multiple heating zones in many differentconfigurations. The controller may therefore be configured to preventall the heater electrodes distributed across one or more plates beingpowered at the same time (or for only short periods of time).

As previously discussed, we generally consider a current draw ‘I’ tocorrespond to the current draw necessary to power a heater electrodeheating an entire heater plate. Thus, in embodiments having multipleheating zones on a heater plate, one electrode in each zone may beconsidered to draw (for the purposes of comparison only), a portion ofcurrent draw ‘I’. In an embodiment having two heating zones on eachheater plate, i.e. four heating zones in total, each zone may beconsidered to draw a current of 0.5I (presuming the resistances aregenerally the same). A maximum preferred current draw ‘I’ may thereforecorrespond to powering two zones simultaneously. Any two zones: both onthe same plate, or one on each plate may be simultaneously powered. Thismeans that in the event a quantity of hair is placed on only one sectionof the heater plates, such that only one zone needs to be powered toretain the desired operating temperature, then opposing zones on twoheater plates may be simultaneously powered whilst staying within thepreferred current draw limit (‘I’) to prevent the battery sourceoverheating.

It will be appreciated that the maximum preferred current draw toprevent the battery source overheating may not be ‘I’, it may instead behigher or lower than this, Therefore, in some embodiments it may then bepossible to power different combinations and numbers of heating zonessimultaneously without the battery source overheating. By way ofexample, in an embodiment having two heater zones on each of two heaterplates, given a preferred maximum current draw of ‘1.5I’, it may then bepossible to power three heater zones simultaneously whilst stayingwithin the preferred current draw limit.

Table 1 below shows exemplary combinations of the maximum zones that maybe powered at any one time. The ‘nominal current draw’ column providesexamples of the nominal current draw limit, defined in multiples of thecurrent draw of one heater arranged to heat an entire heater plate.Accordingly, for the purposes of this illustrative example, the currentdraw of two heater electrodes, each heating half of a heater plate, isdeemed the same as one heater element powering an entire heater plate.It will however be appreciated that in practice the current draw may bedifferent.

Zones per Number of plates Nominal Zones powered at plate in appliancedraw ‘I’ any one time 1 2 1 1 2 2 1 2 3 2 1 3 1 2 1.5 1 2 2 1.5 3 3 21.5 3

Returning now to the embodiments shown in FIG. 5 having one heaterelectrode in each of two plates, in a second control mode themicrocontroller may allow periods of overlap in which both heaters arepowered simultaneously to heat up both heater plates at the same time.Given the nominal preferred current draw of ‘I’, limited periods of ahigher current draw may be permitted, so long as these higher currentdraw periods are interleaved with rest periods in which the nominalpreferred current draw is not exceeded. So as to prevent overheating,the duration of overlap may be limited by the microcontroller/controlmeans. The microcontroller/control means may be configured to limit thisoverlap to a predetermined duration within a fixed period of time basedon predetermined characteristics of the battery. The microcontroller maypermit, for example, simultaneous heating to only take place during theinitial heat up from power on, then revert to the first mode ofoperation. In other words, the controller may be configured to limit aduration in which the two heater electrodes are simultaneously powerableby the power source. This may also be dependent on the batterytemperature or ambient temperature.

In an enhancement to the second control mode the overlap control may bevariable, being controlled, for example, in response to feedback from abattery temperature sense signal 5051 as depicted in FIG. 5. In thisvariant, the microcontroller may then actively monitor the temperatureof the battery source, controlling the permissible overlap in which thenominal preferred current draw may be exceeded in response to thetemperature of the battery source. This may be useful to allow bothheaters (based on an embodiment have one heater element in each heaterplate) to be driven simultaneously from cold at power on, with themicrocontroller then disabling any overlap in heating once the heatersare first up to temperature.

By way of example, FIGS. 6c and 6d show a graph for each heater in anappliance having two heaters where overlap is permitted. In the firstphase, the battery temperature is within the preferred operating rangeand so the microcontroller is configured to operate in the secondcontrol mode with periods in which both heaters are heatedsimultaneously. In the second phase, the battery temperature sensor maysense the temperature approaching (or exceeding) a threshold temperaturewhich results in the microcontroller changing to the first control ofoperation in which the heaters are powered in anti-phase. Themicrocontroller may then optionally return to the second control modewhen the temperature of the battery source drops. The dotted linesbetween the Figures show regions of overlap in heaters being powered inthe first phase.

The second technique may also be implemented for embodiments havingmultiple heating zones on one or more of the heater plates.Incorporating the second technique, the microcontroller may then permitvarious combinations of zones to be heated simultaneously as previouslydescribed, with periods in which the nominal preferred current draw isexceeded by powering further heating zones for a limited period of time.This means that the controller may be configured to limit a duration inwhich at least two or more of the heater electrodes are simultaneouslypowerable by the battery power source.

FIG. 7 shows a cross-sectional view of an illustrative embodiment of anarm 700 of a hair styling appliance. The arm 700 comprises an outercasing 712 to which other components of the hair styling appliance aresecured. A heater element 704 is positioned on heater plate 702 to forma hair styling heater assembly. The hair styling heater assembly is thenretained on the arm by the use of a thermally insulating supportstructure 714.

The heater plate 702 comprises a styling surface 715 on one side thatcontacts the hair to be styled during use. On the other side of theheater plate two L-shaped recesses 709 a, 709 b provide sockets forsecurely fixing the hair styling heater assembly to the thermallyinsulating support structure 714.

The thermally insulating support structure 714 is formed from insulatingmaterial and may, for example, be constructed from a similar material tothe casing. The support structure 714 comprises a pair of L-shapedprojections 708 a, 708 b arranged to fit into the 709 a, 709 b recessesin the heater plate 702 and couple the heater plate and supportstructure together. To allow the projections to fit into the recesses,they may have a small degree of flex such that then can snap-fit intothe recesses, thereby securely fixing the heater plate and supportstructure together. It will be appreciated however that other means forcoupling the hair styling heater assembly and the support structure arepossible, and the example shown in FIG. 15 is purely illustrative of oneway of doing so.

To secure the support structure 714 to the casing, sprinted members 710a and 710 b are used. These are secured at one end to the casing and atthe other end to the support structure. In the illustrative embodimentshown in FIG. 15, compression springs are used which bias the heaterassembly and support structure away from the arm. These allow the heaterplate to move relative to the casing during styling, allowing the platesto retain contact with varying thicknesses and changes in the profile ofhair clamped between opposing pairs of styling surfaces on the heaterplates. It will be appreciated that various other arrangements may beused that provide allow for movement of the heater plates.

This heater assembly arrangement provides several advantages:

-   -   1. Firstly, it reduces the width of the outer casing needed to        retain the hair styling heater assembly as no retaining lugs or        fixings are now needed at the sides of the heater assembly.    -   2. Secondly, with no protrusions extending to one or more sides        of the heater plate 702, the widest part of the heater plate is        the styling surface 715. Such an arrangement is particularly        advantageous during manufacturing as it allows the heater plates        to be closely packed, with no or minimal gap between them. This        allows a large number of styling surfaces to be screen printed,        as if they were one large surface, improving the efficiency of        the printing process.

The skilled person will appreciate that the techniques we have describedabove may be employed for a range of hair styling appliances including,but not limited to, a hair straightener, a hair crimping device, and ahair curler. The skilled person would also appreciate that features frommany of the embodiments are interchangeable and not limited to thespecific embodiment they are described in relation to.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

1. A hair styling apparatus comprising: a plurality of heater electrodeswhich heat one or more hair styling heaters, the plurality of heaterelectrodes comprising a first subset and a second subset; a power sourcefor powering the plurality of heater electrodes and a controllerconfigured to control powering of the plurality of heater electrodesfrom the power source; wherein, in a first mode of operation, thecontroller is configured to control the power source so that the firstand second subsets of the plurality of heater electrodes are notsimultaneously powered and are powered in a time interleaved mannermultiple times per second.
 2. (canceled)
 3. A hair styling apparatusaccording to claim 1, wherein the controller is configured to select thefirst and second subsets so that a total current drawn by each of thefirst and second subsets is below a predetermined current threshold. 4.A hair styling apparatus according to claim 1, wherein no heaterelectrodes are selected for both the first and second sub sets.
 5. Ahair styling apparatus according to claim 4, wherein the controller isconfigured to switch the power source between the first and secondsubsets.
 6. A hair styling apparatus according to claim 5, wherein thecontroller is configured to switch between each subset multiple timesper second.
 7. A hair styling apparatus according to claim 1, whereinthe controller is configured to alternate between the first mode ofoperation and a second mode of operation in which the first and secondsubsets of the plurality of heater electrodes are simultaneouslypowered.
 8. A hair styling apparatus according to claim 7, wherein thecontroller is configured to switch from the second mode of operation tothe first mode of operation in response to a control signal.
 9. A hairstyling apparatus according to claim 8, wherein the control signal isthat a predetermined amount of time in the second mode of operation haselapsed.
 10. A hair styling apparatus according to claim 7, wherein thepower source is a battery source and the hair styling apparatus furthercomprises a battery temperature sensor which senses the temperature ofthe battery source and which sends a battery temperature sense signal tothe controller.
 11. A hair styling apparatus according to claim 10,wherein the battery temperature sense signal is compared to a batterytemperature threshold and the control signal is generated when thebattery temperature sense signal is greater than the battery temperaturethreshold.
 12. A hair styling apparatus according to claim 11, whereinthe battery temperature threshold is in the range of 60 to 80 degreesC., more preferably 70 degrees C.
 13. A hair styling apparatus accordingto claim 7, further comprising a temperature sensor sensing ambienttemperature; and said control signal is generated when the sensedambient temperature is below a threshold ambient temperature.
 14. A hairstyling apparatus according to claim 13, wherein the ambient temperaturethreshold is in the range of 25-35 degrees C.
 15. A hair stylingapparatus according to claim 1, comprising a first arm having a firstcontacting surface and a second arm having a second contacting surface,wherein the arms are moveable between a closed position in which thefirst and second contacting surfaces are adjacent and an open positionin which the first and second contacting surfaces are spaced apart. 16.A hair styling apparatus according to claim 15, wherein the first armcomprises a first hair styling heater having a plurality of heaterelectrodes.
 17. A hair styling apparatus according to claim 15, whereinthe first arm comprises a first hair styling heater and the second armcomprises a second hair styling heater and each hair styling heatercomprises at least one heater electrode.
 18. A hair styling apparatusaccording to claim 1, comprising a touch sensitive switch configured toenable or disable the hair styling apparatus.
 19. A hair stylingapparatus according to claim 18, comprising a first arm having a firstcontacting surface and a second arm having a second contacting surface,wherein the arms are moveable between a closed position in which thefirst and second contacting surfaces are adjacent and an open positionin which the first and second contacting surfaces are spaced apart,wherein the touch sensitive switch is located on the first or secondcontacting surface.
 20. A hair styling apparatus according to claim 18,wherein when the arms are in the closed position, the touch sensitiveswitch is deactivated.
 21. A hair styling apparatus according to claim18, wherein the controller is configured to determine when a user hasactivated said touch sensitive switch for at least a predeterminedduration of time and enable or disable the hair styling apparatusresponsive to said determining.
 22. A hair styling apparatus accordingto claim 1, wherein the or each heater comprises a heater plate which ismounted on a thermally insulating support structure.
 23. A hair stylingapparatus according to claim 22, wherein the heater plate comprises atleast one recess which cooperates with a corresponding projection on thethermally insulating support structure.
 24. A hair styling apparatusaccording to claim 22, wherein the thermally insulating supportstructure is resiliently mounted within an arm of the hair stylingapparatus.
 25. A hair styling apparatus according to claim 1, whereinthe power source is a battery source.
 26. A hair styling apparatusaccording to claim 1, wherein, in the first mode of operation, thecontroller is configured to control the powering of the heaterelectrodes using predefined heating cycles during one or more of whichpower is provided, at different times, to the first and second subsetsof the plurality of heaters, and preferably wherein the heating cycleshave a duration of between 100 μs and 500 ms.
 27. A hair stylingapparatus according to claim 1, wherein the controller is configured touse pulse width modulation, (PWM), power control to control the deliveryof power to the first and second subsets such that, during the firstmode of operation, the controller is configured to generate powercontrol signals for the first and second subsets to cause power to bedelivered to the first and second subsets at different times within eachof one or more heating cycles of the PWM power control.
 28. A method ofcontrolling a hair styling apparatus comprising a plurality of heaterelectrodes which heat one or more hair styling heaters, the plurality ofheaters comprising a first subset and a second subset; the methodcomprising: controlling powering of the heater electrodes so that thefirst and second subsets of the plurality of heaters are notsimultaneously powered and are powered in a time interleaved mannermultiple times per second.
 29. A controller for a hair stylingappliance, wherein the controller configured is configured to implementthe method of claim 28.